Filled impulse lines
An alternate method for isolating a pressure-sensing instrument from direct contact with process fluid is to either fill or purge the impulse lines with a harmless fluid. Filling impulse tubes with a static fluid works when gravity is able to keep the fill fluid in place, such as in this example of a pressure transmitter connected to a water pipe by a glycerin-filled impulse line:
A reason someone might do this is for freeze protection, since glycerin freezes at a lower temperature than water. If the impulse line were filled with process water, it might freeze solid in cold weather conditions (the water in the pipe cannot freeze so long as it is forced to flow). The greater density of glycerin keeps it placed in the impulse line, below the process water line. A fill valve is provided near the transmitter so a technician may re-fill the impulse line with glycerin (using a hand pump) if ever needed.
As with a remote diaphragm, a filled impulse line will generate its own pressure proportional to the height difference between the point of process connection and the pressure-sensing element. If the height difference is substantial, the pressure offset resulting from this difference in elevation will require compensation by means of an intentional “zero shift” of the pressure instrument when it is calibrated.
Purged impulse lines
Continuous purge of an impulse line is an option when the line is prone to plugging. Consider this example, where pressure is measured at the bottom of a sedimentation vessel:
A continuous flow of clean water enters through a “purge valve” and flows through the impulse line, keeping it clear of sediment while still allowing the pressure instrument to sense pressure at the bottom of the vessel. A check valve guards against reverse flow through the purge line, in case process fluid pressure ever exceeds purge supply pressure. Purged systems are very useful, but a few details are necessary to consider before deciding to implement such a strategy:
- How reliable is the supply of purge fluid? If this stops for any reason, the impulse line may plug!
- Is the purge fluid supply pressure guaranteed to exceed the process pressure at all times, for proper direction of purge flow?
- What options exist for purge fluids that will not adversely react with the process?
- What options exist for purge fluids that will not contaminate the process?
- How expensive will it be to maintain this constant flow of purge fluid into the process?
Also, it is important to limit the flow of purge fluid to a rate that will not create a falsely high pressure measurement due to restrictive pressure drop across the length of the impulse line, yet flow freely enough to achieve the goal of plug prevention. In many installations, a visual flow indicator is installed in the purge line to facilitate optimum purge flow adjustment. Such flow indicators are also helpful for troubleshooting, as they will indicate if anything happens to stop the purge flow. In the previous example, the purge fluid was clean water. Many options exist for purge fluids other than water, though. Gases such as air, nitrogen, or carbon dioxide are often used in purged systems, for both gas and liquid process applications.
Purged impulse lines, just like filled lines and diaphragm-isolated lines, will generate hydrostatic pressure with vertical height. If the purge fluid is a liquid, this elevation-dependent pressure may be an offset to include in the instrument’s calibration. If the purge fluid is a gas (such as air), however, any height difference may be ignored because the density of the gas is negligible.